Look for Rust pre 1977.. Look for CIS issues on 74-83 911's... Given age, many will have neglected suspensions (bushings, shocks etc). Headliners will be questionable. Some electronics may not work. AC won't be effective even if it does "work".
Is there a particular vintage you are looking for? Would you prefer a stock or modded car? What is your goal for the finished project?
imho, most 911's are $20K cars.. whether you spend $9K on basket or spend $12-14K for a decent "driver" or 20 (or even $20k+) for a really nice example.